TRACKING RADARS / LOBE SWITCHING / CONICAL & HELICAL SCANNING

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Würzburg D in use. The quirl conical scanning antenna is prominent.
Würzburg D in use. The quirl conical scanning antenna is prominent.

The search radar / height finder radar combination was fine for vectoring fighters against intruders, but antiaircraft guns needed a single radar that could zero precisely in on a target and track it.

As mentioned earlier, a radar’s accuracy is a function of the angular width of its beam. A radar beam can be thought of as something like a radar “spotlight”, with a very narrow spotlight beam able to more precisely pin down the direction of a distant target than a broad one.

There was a way to get accuracy much better than the actual width of the beam. Radar antennas typically emit electromagnetic radiation in the form of a teardrop-shaped “lobe”, tapered at the sides and broad at the tip. Trying to pin down a target in a single broad lobe is troublesome — but suppose the radar transmitter actually has two antennas, toed out slightly relative to their mutual centerline, and the transmitter alternates sending pulses, sending a pulse with one and then the other consecutively. The radar operator can then steer this antenna array until the alternating returns are the same size, meaning the target is on the centerline. Since the edges of the lobes are relatively sharp, this allows relatively precise location of the target. The error signals provided by the difference in the two lobes can be used to control servo motors that guide the radar along the track of the target automatically: if the signal is stronger in one lobe than the other, the antenna is steered in the direction of the stronger lobe until the two signals balance.

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This scheme is known as “lobe switching” or just “lobing” and it is a form of what is called “angle tracking”. Some early anti-aircraft radars used horizontal and vertical lobe switching to target intruders. A good example was the US “SCR-268” anti-aircraft radar, which was developed alongside the SCR-270 search radar mentioned in the previous chapter and shared some of the same technology. The SCR-268 operated at 100 MHz / 1.5 meters. It was somewhat clumsy-looking, featuring a transmit antenna, a vertical lobing receiver antenna, and a horizontal lobing receiver antenna, all mounted together on a single gun-type mount. The transmit antenna was in the form of a 4 x 4 array of dipoles; the vertical receiver antenna was a 2 x 6 rectangular array of dipoles, mounted with its long axis vertical on the right; and horizontal receiver antenna was a 6-by-4 array of dipoles, mounted with its short axis vertical on the left. The SCR-268 had a beam width of 2 degrees in both the horizontal and vertical directions, and a maximum range of 36 kilometers (23 miles). As awkward as it looked, the SCR-268 was actually a fairly good piece of gear by the standards of the time, and would remain in first-line service for gun laying and searchlight direction late into the war, its retirement mostly being driven by the fact that the Germans had figured out how to jam it.

Some shipboard radars used to direct naval guns for firing on surface targets got by with only horizontal lobe switching.

* Anti-aircraft radars were then refined to a more sophisticated scheme for lobe switching, known as “conical scanning”. This involved a parabolic dish antenna with a radio “feed” element that was slightly offset from the centerline. The feed element was rotated at a low rate to generate pulses slightly skewed from the centerline, with the dish steered until the returns were all equal.

Some of these radars also had “helical scanning”, which sounds the same but was actually something different, meaning that the entire radar dish spun around in a helical pattern while it was searching for a target, something like the way a height-finder radar nodded up and down. Once the targeting radar found a target, it stopped helical scanning and used conical scanning to pin down its precise location.

Originally, anti-aircraft targeting radars simply gave aim points for anti-aircraft guns. The scheme was quickly improved so that the error signals from the tracking radar not only steered the radar antenna, they steered the gun automatically as well. Since the gun had to “lead” the target to score a hit, it couldn’t point in exactly the same direction as the radar antenna, with an analog computer in the loop calculating the proper lead for the gun. The result was an improvement in lethality by an order of magnitude or more. The classic example of such a gun-laying radar was the US “SCR-584”, which was a microwave set with a circular parabolic dish using helical and conical scanning. It was linked to a heavy antiaircraft gun through an analog computer system.

The technology has been refined since the war, one of the prominent examples being the well-known and highly effective Soviet-Russian ZSU-23-4 “Shilka” tracked antiaircraft vehicle, with quadruple 23 millimeter automatic cannon in a turret mount, guided with speed and accuracy by an automatic radar fire-control system.